I'm looking for a leaf filter/separator or something else for the downpipe (also homemade, if that helps) that is quick and easy to clean/change and doesn't need to change/cut anything on the existing pipe or anything (access to the beginning of the pipe is hard to come by). Are there any other options?
In front of the last hole you can reach, you can bend a hexagonal mesh called a rabbit screen.
Helps on my balcony against mice striving up and leaves striving down the drain.
I would consider replacing part of the downpipe with an appropriate dirt collector part. Perhaps hook up a rain barrel to it.
You should be able to find a proper fit under the mudguards or alternate leaf separators – but you can't install them without modifying the downpipe.